1. Field of the Invention
The present invention relates to a display device using a thin film transistor.
2. Description of the Related Art
In a display device formed using an inexpensive glass substrate, a proportion of a region in the periphery of a pixel portion (a frame region), which is used for mounting, to a substrate is increased as resolution is increased; thus, reduction in size of the display device tends to be prevented. Therefore, it has been considered that a method in which a driver circuit formed using a single crystalline semiconductor substrate is mounted on a glass substrate has limitations, and a technique for forming a driver circuit and a pixel portion over the same glass substrate, that is, so-called system-on-panel has been emphasized. By realization of system-on-panel, the number of pins for connecting a driver circuit and a pixel portion can be reduced, and problems such as reduction in yield due to defective connection of a driver circuit and a pixel portion and low mechanical strength in a connection portion using pins, which have been caused when the driver circuit using a semiconductor substrate is mounted on a glass substrate, can be avoided. Further, by realization of system-on-panel, reduction in cost due to reduction in the number of assembly steps and inspection steps can be achieved in addition to reduction in size of the display device.
Typical driver circuits included in the display device are a scan line driver circuit and a signal line driver circuit. A plurality of pixels in one line (or in a plurality of lines in some cases) are selected by the scan line driver circuit. Input of a video signal to the pixel in the selected line is controlled by the signal line driver circuit.
As for the signal line driver circuit and the scan line driver circuit, driving frequency of the scan line driver circuit is easily suppressed as compared with that of the signal line driver circuit; thus, it can be said that the scan line driver circuit can be comparatively easily formed over a glass substrate. Reference 1 discloses a technique that a scan line driver circuit and a pixel portion are formed on a glass substrate with a transistor using an amorphous semiconductor (Reference 1: YongSoon Lee et al., “Advanced TFT-LCD Data Line Reduction Method”, SOCIETY FOR INFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, Volume XXXVII, pp. 1083-1086, 2006).